Batteries using various cell technologies (cell types) have been developed and are in widespread use. Two of the most common cell types are alkaline and nickel-cadmium (Ni-Cd). Other cell types include nickel-metal-hydride and various lithium technologies. In many applications., batteries of differing cell types are interchangeable. For example, in many household electronic products such as calculators, palmtop computers and the like, both alkaline and Ni-Cd type batteries can be used.
In some circumstances, however, it is necessary to differentiate between the cell types of the batteries being used in a particular application. For example, in some palmtop computers, an end-of-life or state-of-charge procedure is used to gauge the remaining energy in the battery and provide an indication of same to the user. Typically, these procedures are dependent on the voltage characteristic of the battery over its discharge cycle. Because the voltage characteristic of batteries differs between cell types, the cell type of the battery must be known to accurately gauge the remaining battery energy. If a battery is compared to a voltage characteristic of another cell type, the measurement of the remaining energy will be in error. Improper cell type usage could result in the diagnosis of a fully charged battery as having insufficient remaining charge. The user might prematurely discard or unnecessarily recharge such a misdiagnosed battery. Improper cell type usage could also result in failing to diagnose the impending end of life of a battery with little remaining charge. The user, relying on such diagnosis, could lose valuable data on the eventual failure of the battery.
Cell type differentiation is also critical to battery recharging. Some cell types, such as Ni-Cd, are rechargeable, while others, such as alkaline, are not. If a recharging current is applied to a non-rechargeable battery, the temperature of the battery increases rapidly as does the internal pressure. Eventually, if recharging is continued, the battery will vent, resulting in leakage of caustic chemicals from the battery and possibly an explosion.
Currently, many battery operated products simply rely on the user as directed by an owner's manual or warnings attached to the battery operated product to select and employ-batteries of the proper cell type. Some palmtop computers may even employ user interrogation to identify the cell type prior to gauging the battery's state-of-charge. However, considering the possible consequences of improper cell type usage, it is desirable to have a system to protect the unwary user of a battery operated product from using batteries of the wrong cell type.
Manufacturers of battery operated products have also relied on mechanical lock-out schemes to protect against improper cell type usage. Typical mechanical lock-out devices protect against improper cell type usage by preventing installation of batteries that do not have a particular size, shape or other mechanical feature. Only batteries having the particular mechanical feature can be installed. Typically, the mechanical feature is of non-standard design to ensure that only batteries of the desired cell type have that mechanical feature. However, the use of non-standard batteries usually results in limited availability of suitable batteries to the public. Products employing these schemes may therefore have difficulty in gaining public acceptance. Further, lock-out schemes are not useful to palmtop computer state-of-charge gauges, where the use of interchangeable cell types is desired but the particular cell type in use must be known.
The ability to identify the cell type of a battery while the battery is in use is therefore needed. The present invention provides an apparatus and method for identifying the cell type of a battery while it is in circuit and operating. The invention uses the series resistance of the battery to identify its cell type.
Batteries of different cell types have different discharge characteristics for a particular method of discharge. Most electronics systems use a pulsed, constant power discharge method (i.e., a constant power is intermittently drawn from the battery). Other discharge methods include constant load impedance, constant current, and constant power discharge. It is possible to characterize the behavior of various cell types under a specific discharge method and specific end use. The characteristic behavior of a cell type in a specific use can then be applied to differentiate between cell types.
One particular cell discharge characteristic, the series resistance, is distinct for various cell types under pulsed, constant discharge usage. For example, the series resistance of alkaline batteries falls within a certain range through its life cycle. The series resistance of Ni-Cd batteries, however, falls within a second range of values that, for the most part, is distinct from that of alkaline batteries. The present invention uses this series resistance behavior of batteries to distinguish the cell type.
In accordance with the method of the present invention, the series resistance behaviors of the cell types that may be used in a particular application are characterized to establish the range of values exhibited by the cell types over their lifetimes. Thereafter, the series resistance of a battery in that application is measured and compared to the various ranges associated with the cell types. The battery is then identified as having the cell type associated with the range of values in which its measured series resistance falls.
In the case of some cell types, however, the series resistances of the cell types are not distinct over their full discharge cycle. For example, alkaline and Ni-Cd cell types both exhibit series resistances in a certain intermediate range of values in different portions of their life cycles. In such cases, it is also necessary to use a second discharge characteristic, such as battery voltage, to differentiate between cell types. When the cell types exhibit series resistance values in the intermediate range, the battery voltage or other discharge characteristic of the cell types fall into differing, distinct ranges. Thus, in some embodiments of the invention, the battery voltage or other discharge characteristic of the battery is also measured and compared along with the series resistance to established cell type behaviors to identify the cell type of the battery.
Also in accordance with the present invention, an apparatus for use in an electronic device for identifying the cell type of a battery is provided. The apparatus comprises a battery testing circuit and a cell type identification circuit. In one embodiment of the present invention, the battery test circuit measures the series resistance and voltage of the battery. The testing circuit measures the series resistance of a battery by pulsing the battery with a current pulse of known magnitude and measuring the change in the battery voltage produced by the pulse. The series resistance is the ratio of the voltage change to the magnitude of the current pulse. The testing circuit measures the battery voltage by sampling the voltage and digitizing the sampled voltage.
The cell type identification circuit is preferably embodied in a microprocessor programmed to execute the method of the present invention, but may also be embodied in dedicated hardware. The identification circuit receives the series resistance and voltage measurements from the testing circuit and compares the measurements to established ranges of battery behavior for the cell types. The identification circuit then identifies the battery as being of the cell type associated with the range in which the measurements lie.
Additional features and advantages of the invention will be made apparent from the following detailed description of a preferred embodiment, which proceeds with reference to the accompanying drawings.